This invention relates in general to an imaging system and, more particularly, to an electrostatographic material.
Electrostatography is best exemplified by the process of xerography as first described in U.S. Pat. No. 2,297,691 to C. F. Carlson. In this process a photoconductor is first provided with a uniform electrostatic charge over its surface and is then exposed to an image of activating electromagnetic radiation which selectively dissipates the charge in illuminated areas of the photoconductor while the charge in the non-illuminated areas is retained thereby forming a latent electrostatic image. This latent electrostatic latent image is then developed or made visible by the deposition of finely-divided electroscopic marking particles referred to in the art as "toner." The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electroscopic image. This powdered image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by fusing. Instead of forming latent images by uniformly charging a photoconductive layer and the exposing the layer to a light and shadow image, an image may be formed by directly charging the layer or an insulating member in image configuration. The powder image may be fixed to the imaging layer if elimination of the powder image transfer step is desired. Other suitable means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying the electroscopic particles to the latent electrostatic image to be developed. One well-known commercial method for developing electrostatic images in the "cascade" process disclosed by L. E. Walkup in U.S. Pat. No. 2,618,551 and E. N. Wise in U.S. Pat. No. 2,618,552. In this method a developer material comprising relatively large carrier beads having fine toner particles electrostatically coated thereon is conveyed to or rolled or cascaded across the electrostatic image bearing surface. The composition of the carrier particles is so chosen as to triboelectrically charge the toner particles to the desired polarity. As the image cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of a latent image and are not deposited on the uncharged or background portion of the image. Most of the toner particles accidentally deposited in the background areas are removed by the rolling carrier, due apparently to the greater electrostatic attraction between the toner and carrier than between the toner and the discharged background. The carrier and excess toner are then recycled.
In most commercial processes the cascade technique is carried out in automatic machines. In these machines small buckets on an endless belt conveyor scoop the developer mixture comprising relatively large carrier beads and smaller toner particles and convey it to a point above an electrostatic image bearing surface where the developer mixture is allowed to fall and roll by gravity across the image bearing surface. The carrier beads along with any unused toner particles are then returned to the sump for recycling through the developing system. Small quantities of toner material are periodically added to the developer mixture to compensate for the toner depleted during the development process. This process is repeated for each copy produced in the machine and is ordinarily repeated many thousands of times during the usable life of the developer mixture. It is apparent that in this process, as well as in other development techniques, the developer mixture is subjected to a great deal of mechanical attrition which tends to degrade both the toner and carrier particles. This degradation, of course, occurs primarily as a result of shear and impact forces due to the tumbling of the developer mixture on the image bearing plate and the movement of the bucket conveyor through the developer material in the sump.
Another method of developing electrostatic images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and megnetic carrier particles are carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier into a brush-like configuration. This "magnetic brush" is engaged with the electrostatic image-bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction.
In prior art processes both coated and uncoated carrier beads were employed with degrees of success. Coated carrier beads are subject to deterioration or degradation characterized by the separation of portions of or the entire carrier coating from the carrier core. The separation may be in the form of chips, flakes, or entire layers and is primarily caused by poorly adhering coating materials which fail upon impact and abrasive contact with machine parts and other carrier particles. Carriers having coatings tend to chip and otherwise separate from the carrier core and must be frequently replaced, thereby increasing expense and consuming time. Print deletion and poor print quality occur when carrier particles having damaged coatings are not replaced. Fines and grit formed by the carrier coating disintegration tend to drift and form unwanted deposits on critical machine parts. In addition, the triboelectric properties of the carrier material varies with deterioration of the coating resulting in poor print quality.
Uncoated carrier beads on the other hand have three main deficiencies. First, often they lack the weight required to insure against adherence of the granular carrier material to the charged plate. Desirably, the specific gravity of the carrier material should be between about 3 and about 8.9. Heavier carrier bead materials cause impact damage to the surface of the image bearing layer. Secondly, the prior art uncoated carrier materials lacked the triboelectric qualities required of an electrostatographic material. Problems encountered when carrier materials lack these properties are set out in the following discussion. In the reproduction of high contrast copies such as letters, tracings and the like, it is desirable to select the electroscopic powder and carrier materials so that their mutual electrification is relatively large; the degree of such electrification being determined in most cases by the distance between their relative positions in the triboelectric series. However, when otherwise compatible electroscopic powder and carrier materials are removed from each other in the triboelectric series by too great a distance, the resulting images are very faint because the attractive forces between the carrier and toner particles compete with the attractive forces between the latent electrostatic image and the toner particles. Although the image density described in the immediately preceding sentence may be improved by increasing the toner concentration in the developer mixture, undesirably high background toner deposition as well as increased toner impaction and agglomeration is encountered when the developer mixture is overtoned. The initial electrostatographic plate charge may be increased to improve the density of the deposited powdered image but the plate charge would ordinarilly have to be excessively high in order to attract the electroscopic powder away from the carrier particle. Excessively high electrostatographic plate charges are not only undesirable because of the high power consumption necessary to maintain the electrostatographic plate at high potentials but also because a high potential causes the carrier particles to adhere to the electrostatographic plate surface rather than merely roll across and off the electrostatographic plate surface. Print deletion and massive carryover of carrier particles often occur when carrier particles adhere to reusable electrostatographic imaging surfaces. Massive carrier carryover problems are particularly acute when the developer is employed in solid area coverage machines where excessive quantities of toner particles are removed from carrier particles thereby leaving many carrier particles substantially bare of toner particles. Further, adherence of carrier particles to reusable electrostatographic imaging surfaces promotes the formation of undesirable scratches on the surfaces during image transfer and surface cleaning operations. It is, therefore, apparent that many material which otherwise have suitable properties for employment as carrier particles are unsuitable because they possess too high a triboelectric value. Desirably, the triboelectric value for conventional electrostatography measured in micro-coulombs per gram of toner should be between 8 and 30.
Finally, the triboelectric value of a carrier material should not be significantly affected by ambient humidity conditions since such affect would destroy print quality at higher humidities and complicate machine design and operation, many prior art uncoated materials were never commercially successful because of their great humidity sensitivity.